Initialize unit tests with empty local devices

Before: HdmiControlService is initialized with the local devices
specified in HdmiProperties.cec_device_types(). This results in
different behavior for different DUTs.

After: HdmiControlService is initialized with empty local
devices, practically ignoring HdmiProperties.cec_device_types().

Bug: 187909454
Test: atest com.android.server.hdmi on Cuttlefish and ADT3

Change-Id: I72c2ae4d0621c728f80ed8d7755719263428e384